A push-push is a switch (as first used on guitars by Yamaha on the SG2000 I believe) where instead of having to grab the knob and pull it up to engage, and push it down to disengage it, you push it down to engage, the knob pops up, and you then push the knob down to disengage it. It's a lot easier to use than a push-pull knob, especially if you have sweaty hands. Though if your hand moves over the and knocks the control knobs a lot when playing, you could possibly operate the switch by accident (although you do need to apply a modicum of downwards pressure to operate them).

They aren't as common as push-pulls, but are normally a very similar price.